The present invention relates in general to audio amplifiers and in particular a method and apparatus for obtaining power supply isolated high power audio.
Audio circuits are well known in the prior art and generally utilize an audio amplifying circuit to drive the audio speaker. The audio amplifier is required to have a power audio stage. Such power amplifier circuits are illustrated in U.S. Pat. Nos. 4,409,559 and 4,651,112.
A typical class AB audio amplifier has an isolated power supply and the audio amplifier. The cost of these two units together is significant in the manufacture of an audio output device. There are many switching power amplifiers that attempt to increase power efficiency over class AB. Such circuits are shown in U.S. Pat. Nos. 4,763,080, 4,517,522, 4,047,120 and 4,453,131. These still need an isolated power supply, unipolar or bipolar.
U.S. Pat. No. 5,986,498 teaches an audio amplifier using two transformers for isolating main voltage from the output circuitry. However it has a few flaws and drawbacks: its embodiment as disclosed involves two switching transistors driven by a transformer and an input signal zero-voltage crossing detector, and a phase-lag network. Its two MOSFETs are one too many. Driving MOSFETs with a transformer is slow and difficult due to its leakage inductance and the need for flux balancing. The zero-voltage crossing of the input signal does not coincide with the zero-voltage crossing of the output voltages due to delays in the modulator, switching delays of the MOSFETs, and rise time of the transformers, and characteristics of the load. Furthermore, a phase lag network reduces the bandwidth of the amplifier and therefore increases its distortion especially for high frequencies. Most importantly, ""498 teaches an amplifier incapable of driving a reactive load which audio speakers normally are, because the rectifiers at its output only allow unidirectional current flow, whereas a reactive load needs to dump its reactive energy to the source during portions of a sine wave.
What is needed is a power supply isolated switch-mode amplifier capable of driving any load, capable of high bandwidth and low distortion in addition to high efficiency, small size and weight, and low cost.
In accordance with the present invention a novel and useful class-N amplifier is herein provided.
This invention is a family of class-N amplifiers where the functions of a power converter such as an AC-to-DC switch-mode converter or a boost converter are integrated into the circuitry of a switching amplifier to produce a high power signal. The output section of a class-N amplifier is inherently electrically isolated from the power source such as rectified AC voltage for personal safety purposes via a high-frequency power transformer. The invention can be briefly described as a combination of a power modulator transmitting modulated power via a power transformer, followed by a synchronous demodulator driving a speaker. The modulator modulates the power supply voltage with pulse-width modulated (PWM) pulses. A power transformer transmits the modulated voltage to a synchronous demodulator, which reconstructs both phase and amplified amplitudes of the input signal, and filters out carrier frequencies to provide power audio signal to a speaker. Feedback circuits force faithful reproduction of the output signal with minimum distortion.
It is therefore an object of the present invention to provide an efficient and simple method and apparatus for amplifying an audio signal into high power signal with high efficiency and with isolation from power source.
Another object of the present invention is to provide a method and apparatus for amplifying and driving an audio signal into any reactive load with high efficiency.
A further object of the present invention is to provide a class-N amplifier in which heat generation is substantially reduced during its operation.
Another object of the present invention is to provide a class-N amplifier which is amenable to miniaturization and, thus, is useful in audio systems, computers, and any other devices where minimal heat generation and minimal electromagnetic interference generation are criteria.
Another object of the present invention is to provide a class-N amplifier which possesses a high level of reliability due to low component count.
Further objects and advantages of this invention will become apparent from a consideration of the drawings and ensuing description.